A liquid crystal light valve useful for projecting bright images is discussed in Bleha et al, "Application of the Liquid Crystal Light Valve to Real Time Optical Data Processing", Optical Engineering (The Journal of the Society of Photo-optical Instrumentation Engineers), July/August 1978, Vol. 17, No. 4, pages 371-384, the disclosure of which is incorporated by reference in its entirety into this application. The light valve described in the foregoing publication includes a twisted nematic liquid crystal which controls the transmission of light from a polarizer to an analyzer having its optical axis oriented perpendicular to the optical axis of the polarizer. In the null state, the projection light beam is transmitted through the liquid crystal, the angle of polarization of the light beam being rotated in one direction and then rotated in the opposite direction during the return trip through the liquid crystal so that it cannot pass through the analyzer. In the "on" state, an applied electric field across the thickness of the liquid crystal causes the optical axis of the liquid crystal to be tilted toward the direction of the electrical field. This introduces a phase change between the polarization components of the light which are parallel and perpendicular, respectively, to the optical axis of the liquid crystal. This phenomenon, occurring during the "on" state is termed "birefringence".
As a result, linearly polarized light incident on the liquid crystal is reflected back through the liquid crystal as elliptically polarized light which includes a component transmitted through the analyzer.
The quality of the image produced by such a light valve depends in large part upon the contrast ratio between the null state and the "on" state of the liquid crystal. Specifically, the less light transmitted through the liquid crystal in the null state, the greater the contrast ratio. A peculiar problem arises when more than one wavelength of light is to be controlled by the liquid crystal. For example, if the liquid crystal light valve is to be used in a color video system, it would be desirable to project (for example) red, blue and green light onto the liquid crystal. The incident light beam is resolved into two components in the liquid crystal having polarization electric field vectors which are parallel and perpendicular, respectively, to the major optical axis of the liquid crystal whenever the polarization direction of the incident light and the optical axis of the liquid crystal are not parallel. The problem is that a phase delay is introduced in the liquid crystal between the two components. This latter phase delay, occurring during the null state, shall be called null state birefringence and arises because the liquid crystal exhibits different refractive indices to the two light components so that the two components travel through the liquid crystal at different velocities. The light transmitted through the liquid crystal will be at least slightly elliptically polarized due to the phase delay between the two components so that, even in the null state, there will be a small component of light which will pass through the analyzer, thus making it impossible to achieve perfect darkness in the null state, thereby degrading the contrast ratio of the system. The birefringence discussed here is chromatic because it introduces difference phase delays for different wavelengths or colors, and therefore is an especially significant problem when more than one wavelength or color of light is incident on the liquid crystal light valve. For example, if the null state birefringence is compensated at one wavelength, it will usually remain uncompensated at other wavelengths, thus making it impossible to achieve perfect darkness in the null state when a plurality of colors is used.